Alkaline zinc galvanic cells are well known in the art and comprise essentially one or more negative polarity zinc electrodes spaced from a counterelectrode (e.g. NiOOH) of opposite polarity and a concentrated (i.e., greater than about 15%) aqueous alkaline electrolyte (e.g. KOH) bridging the space between the electrodes. A microporous separator material physically divides the opposite polarity electrodes from each other to prevent electronic flow while permitting ionic flow therebetween. These cells typically employ a zinc electrode comprising a zinc-rich active material having a conductive substrate embedded therein to both support and provide conductivity throughout the active material. One such cell, for example, is described in Jones U.S. Pat. No. 4,358,517, issued Nov. 9, 1982, is assigned to the assignee of the present invention, and is intended to be incorporated herein by reference. Jones specifically describes a zinc electrode having an active material comprising zinc oxide, calcium hydroxide, Pb.sub.3 O.sub.4, and cellulose fibers supported on a conductive substrate/current collector (i.e., copper).
The substrates for the zinc-rich active material preferably comprise copper for optimum electrode conductivity. So long as metallic zinc is present in the zinc electrode (especially in contact with the copper), the substrate is protected from dissolution in the electrolyte. However, after the zinc electrode has become fully discharged (i.e., by use or self-discharge of the battery) and the zinc has become oxidized, the copper is left unprotected. If the cell remains in this state for a prolonged period of time, the unprotected copper will dissolve in the electrolyte and the performance of the cell deteriorates. In this regard, dissolved copper from the electrolyte deposits in the pores of the separator dividing the electrodes which, depending on its severity, can cause the cell to no longer accept or retain a charge. More specifically, if copper loading of the separator is severe enough the cell can no longer be recharged and must be replaced. If on the other hand, copper loading of the separator is light, the cell may be charged and used immediately (albeit at reduced capacity), but otherwise will rapidly self-discharge and accordingly cannot be left on open circuit stand for any appreciable period of time.
It is the object of this invention to provide an alkaline zinc galvanic cell having improved shelf life, rechargeability, charge retention and capacity retention following complete discharge of the cell's zinc electrodes by means of an electrolyte additive for retarding the dissolution of copper from copper-containing zinc electrodes used therein. This and other objects and advantages of the present invention will be readily apparent from the description thereof which follows.